Iain Sinclair makes a reader feel lucky to be fluent in English and undoubtedly makes translators despair.

His prose reels from travel notes to social critique, from literary history to collegial caricature, lyrical reverie to satirical rant -- sometimes in a single paragraph.

Sinclair's work being hard to classify as fiction or fabrication -- book by book or line by line -- makes it all the harder to market in America, though Granta has bravely put half a dozen of his titles in print here.

"London Orbital" will win Sinclair new admirers, but only among readers who know what a feat it is to write several hundred pages without a lifeless line.

"Nobody can decide how long the road is," Sinclair writes, "somewhere between 117 and 122 miles. By the time you've driven it, you don't care."

As a project symbolic of a political regime, the M25 is a companion to New Labour's boondoggle Millennium Dome, the subject of Sinclair's little book "Sorry Meniscus" and also the end of his "London Orbital" journey.

Both are defining landmarks in what he calls the "psychogeography" of contemporary Britain.

"Convoys took advantage of the M25," Sinclair writes, "which increasingly functioned as an asteroid belt for London's rubble, the unwanted mess of the building boom, the destruction of tower blocks, the frenzied creation of loft- living units along every waterway."

The only way to get a fix on the M25's reality, he realized, was to walk it,

as he had walked inner London to map his myth-drunk urban history, "Lights Out for the Territory."

"The nature of any walk," he writes, "is perpetual revision, voice over voice."

Egham provides one of the book's central epiphanies: a Siebel Corp. building that acts like an architectural mirage, almost erasing all thought of what might go on there. It sets Sinclair on a riff about the political wisdom of New Labour.

"Even the biggest, most ruthless conglomerates were going belly up because they made the mistake of investing in product," he writes. "Manufacture something, anything, and you're dead. . . . Play smart. Do nothing. New Labour (lessons of the Dome fiasco learnt) have it absolutely right: take soundings, soothe your critics, commission reports. Talk in colourless catch phrases. 'Best value. Economically viable, environmentally sound.' But do nothing."

Stylistically, Sinclair crosses and recrosses terrain made familiar by J.G. Ballard and William Burroughs. H.G. Wells and Bram Stoker also crop up, along with lesser-known names, such as the poet Bill Griffiths and novelist Gerald Kersh, who seems to have been an influence on Sinclair.

Sinclair comes up with information so obscure we doubt its factuality, as when he recounts a meeting between Orson Welles and H.G. Wells in a Texas radio studio.

At one point on the orbital walk, Sinclair actually pays a call on Ballard, whose home in Shepperton adjoins the M25, and who is stunned by Sinclair's arrival on foot.

"The hair is long and silvery," Sinclair writes of Ballard, "the skin ivory coloured. Ballard, through his long residence and his riverine hermeticism, has joined the company. He looks and behaves like a magus, like Dr John Dee: modesty of address enlivened by a proper arrogance about how his vision of the world has been confirmed."

Having noted that the driver negotiating the M25 "underwent the sort of voyage towards insanity, breakdown and reintegration that R.D. Laing and the anti-psychiatrists of the Sixties advocated," Sinclair finds on foot that the highway marks a circuit of decommissioned Victorian asylums, variously rebuilt or resold.

The fugue -- the trancelike amnesiac wandering -- becomes a central metaphor for his own investigation, occasioning several bracing ruminations on the subject.

"London Orbital" makes Sinclair's most powerful argument yet on how much contemporary reality opens up only to those willing to follow the lead of language and a great comic ear like his.